With development and widespread use of information processing technology in recent years, miniaturization, slimming-down, and higher performance of electronic devices are promoted and accordingly, semiconductor packages are also on a path to miniaturization. Particularly semiconductor packages of several pins to 100 pins frequently used for mobile terminals or the like change from conventional SOP (Small Out-line Package) or QFP (Quad Flat Package) to smaller non-lead type SON (Small Out-line Non-lead Package) or QFN (Quad Flat Non-lead Package) and more recently to still smaller WCSP (Wafer-level Chip Scale Package).
Common WCSP has a plurality of solder balls formed on the undersurface of the package in a grid pattern and is connected to the substrate electrode by these solder balls. WCSP is the smallest package that cannot be miniaturized any more because the internal semiconductor chip and the package have the same size.
The manufacturing process of packages such as SOP, QFP, SON, and QFN include a process of mounting semiconductor chips as individual pieces after dicing on a lead frame, a process of connecting semiconductor chips by wire bonding, a process of molding using a sealing resin, a process of cutting a lead, and a process of externally plating the lead. On the other hand, the manufacturing process of WCSP includes only a stage before producing semiconductor chips by dicing a wafer, that is, after solder balls are mounted on the surface of the semiconductor wafer, the wafer only needs to be diced into individual pieces and thus, WCSP is characterized by, compared with other packages, extremely high productivity.
In WCSP, the formation of a re-wire by the semi-additive method using electroplating of Cu is required to change the arrangement of electrode pads of a chip to the arrangement of solder balls. The semi-additive method includes five processes: the formation of a seed layer as a cathode for electroplating, the formation of a resist layer obtained by patterning a re-wire shape, Cu plating by electroplating, peeling of the resist layer, and etching of the seed layer. These processes are positioned between BEOL (Back-End Of Line) of a preceding process and a post process in terms of process and dimensions and so are called intermediate processes and equipment close to BEOL is used as mass-production equipment because a wafer process is used.
More specifically, for example, a laminated thin film of Ti and Cu is used to form a seed layer and a sputtering device that forms a metallic thin film on a wafer is used to form the laminated thin film. Also, a coater developer that automatically performs resist coating, baking, development, and cleaning/drying and a stepper exposure device are used to form a resist layer and a sheet-type plating device is used for electroplating. However, though throughput of these devices is high with several thousand wafers/month or more, each of these devices is extremely more expensive than a common post process device such as a wire bonding device and a die bonding device and also requires a larger installation space and so an initial investment amount is large, which makes application of these devices to diversified small-quantity production difficult and also a flexible response to changes of the quantity of production difficult.
Particularly, in an electroplating device that performs Cu plating, three processes of a pretreatment process to remove oxide from the surface of the seed layer, a Cu plating process, and a cleaning/drying process are needed and many devices have a separate treatment bath for each process to prevent mutual contamination between treatment and also an automatic transfer device between baths is needed so that the device tends to increase in size and also to become more expensive. Further in the Cu plating process, when a common copper sulfate plating solution is used, electroplating is normally performed in a current density of 5 A/dm2 or less to maintain good film quality and thickness distribution and a deposition rate obtained in this case is about 1 μm/min at best even if the current efficiency is assumed to be 100% and if the thickness of 10 μm is needed, the time of about 10 min is needed.
Thus, to secure throughput of, for example, 10,000 wafer/month, it is necessary to prepare at least three Cu plating baths that take the longest treatment time, inviting an increasing size and higher costs.
Therefore, various technologies are under development to improve productivity. For example, a technology to perform a plating process safely, reasonably, and swiftly using supercritical or subcritical carbon dioxide is known (see, for example, Patent Documents 1 to 3).
A supercritical fluid is a fluid in a state belonging to none of the solid, liquid, and gas in a phase diagram determined by the temperature and pressure and its main features include high diffusibility, high densities, and zero surface tension so that when compared with conventional processes using a liquid, permeability at a nano level and a high-speed reaction can be expected. For example, the critical point where CO2 enters a supercritical state is 31° C., 7.4 MPa and CO2 is a supercritical fluid exceeding the above temperature or pressure. Supercritical CO2 is not originally mixed with an electrolytic aqueous solution, but is made turbid by adding a surface active agent and applicable to electroplating, which is known as supercritical CO2 emulsion (SCE) electroplating method.
A plating coat formed by the SCE electroplating method has features that leveling properties are high, pinholes are less like to arise, and crystal grains are made finer so that a close-grained film can be formed. A reaction field by the SCE electroplating method is considered to be a field in which micelles of supercritical CO2 are dispersed to flow in an electrolytic solution and an overvoltage of the plating reaction is considered to rise due to desorption of micelles from the cathode surface so that crystal grains become finer. Supercritical CO2 and hydrogen are known to be very miscible and hydrogen generated simultaneously with the deposition of metal is prevented from becoming an air bubble by being dissolved in CO2 so that pinholes are inhibited from arising.
When WCSP is produced, as described above, a floor space to install large-scale production equipment and expensive initial investment are needed and applying WCSP to diversified small-quantity products that are not in line with the equipment and initial investment is practically difficult. Particularly in the Cu plating device, a plurality of treatment baths is needed due to circumstances of a series of processes of plating or to increase throughput, posing a problem of an increasing size or a rising cost of the device.
To reduce the number of plating baths in the plating device to a minimum, it is effective to increase the current density during plating to increase the deposition rate. For example, to describe by taking the above example, by increasing the current density from 5 A/dm2 to 10 A/dm2, the number of Cu plating baths needed for the throughput 10,000 wafer/month can be reduced from three baths to two baths. Further, if the current density can be increased to 20 A/dm2, the number of Cu plating baths can be reduced to the minimum one bath. If the current density is further increased, an activation overvoltage when metal ions in the plating solution are reduced and a metal is deposited rises so that the crystal grain size becomes finer and the surface of a metal deposited film is advantageously smoothed.
On the other hand, a deposited film by plating is desirably formed uniformly on the surface of a plated substrate, but when the current density is increased, the thickness distribution of a deposited film is known to worsen. The thickness distribution of a plating deposited film is almost determined by a primary current distribution determined by an electric field distribution obtained from geometrical conditions such as the shape and arrangement of the cathode and the anode inside the plating bath and in the end determined by a secondary current distribution obtained by correcting the primary current distribution based on an electrochemical reaction on the surface of the cathode. The key factor to determine the secondary current distribution by correcting the primary current distribution is called a Wagner number (Wa) and represented by the following formula.Wa=κ(Δη/Δi)where κ is the specific electric conductance of the plating solution and Δη/Δi is the polarization resistance of a polarization curve of the plating solution. When Wa=0, that is, the polarization is 0, the secondary current distribution is equal to the primary current distribution and with increasing Wa, compared with the primary current distribution, the secondary current distribution is improved to become uniform. The thickness distribution worsens with an increasing current density because Δη/Δi in the above formula decreases with an increasing current density.
When the current density of the cathode is increased, the crystal grain size becomes finer and the surface of a metal deposited film is smoothed, but the polarization resistance decreases and the improvement effect of the secondary current distribution decreases so that convex abnormal growth such as a nodule is more likely to occur. The nodule is considered to grow using a particle or an impurity in the plating solution as a nucleus and once a convex shape is formed on the smooth plated film surface, the electric field distribution is changed and the current is concentrated onto the convex portion. When the polarization resistance is large and the improvement effect of the secondary current distribution is obtained, the current concentration is mitigated, but when the improvement effect is not obtained, the nodule further grows and also the current further concentrates and in the end, a large nodule is considered to be formed.
Further, to be noted when the current density of the cathode is increased is a hydrogen generation reaction on the surface of the cathode. For example, a sulfuric acid solution is used as the electrolyte in a common copper sulfate plating solution and when a potential at which hydrogen is generated is exceeded by increasing the current density, the reaction shown below occurs steeply and the plated film grows while hydrogen is generated intensely so that a porous plated film of undesirable quality having a low density is formed.2H++2e−→H2 
The potential at which the reaction occurs is generally called a hydrogen overpotential and changes depending on pH of the electrolytic solution, the material of the cathode, and the surface state thereof. Particularly when the surface roughness is rough, the hydrogen overpotential decreases significantly. When the cathode current density is a high current density, as described above, the polarization resistance decreases and convex abnormal growth such as a nodule is more likely to arise and thus, there is the possibility of a decreased hydrogen overpotential and lower quality of a plated film in places such as corners of a plated object or a nodule where the current is more likely to concentrate. Therefore, when the current density is increased in the electroplating method, it is necessary to perform plating in a current density corresponding to a voltage sufficiently lower than the hydrogen overpotential and so increasing the deposition rate significantly is practically difficult.
The present invention is made in view of the above circumstances and an electroplating method by which even if the cathode current density is a high current density, the thickness distribution of a plated film is small and convex abnormal growth of a nodule or the like is inhibited and thus, degradation of film quality caused by hydrogen generation is not caused, wherein the deposition rate of plating can significantly be increased when compared with the rate of the conventional plating method and an electroplating device implementing the electroplating method are needed.